1. Field of the Invention
This invention relates to aligning a substrate with respect to another body and in particular to centering a wafer on a chuck in a processing chamber. This invention is especially related to centering a wafer in a chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD) process chamber which is equipped with a carrier ring, which could be an exclusion gas ring for preventing deposition on the backside and peripheral edges of the wafer, particularly equipment for depositing a layer of tungsten, copper, aluminum or another conductor. This invention is also applicable to other types of processing equipment such as physical vapor deposition (PVD) and etching equipment such as that used in reactive ion etching (RIE).
2. Description of Related Art
In semiconductor processing, before a film of material is formed on a wafer, the wafer is customarily placed on a pedestal, frequently referred to as a chuck, in a process chamber. One of the methods of forming a film on the wafer is chemical vapor deposition (CVD), which is a gas reaction process commonly used in the semiconductor industry. The CVD process is based on the thermal, plasma, or thermal and plasma decomposition and reaction of selected gases. The most widely used CVD films are silicon dioxide, silicon nitride, and polysilicon, although a wide variety of other CVD films are suitable for use as insulators, dielectrics, semiconductors, conductors, superconductors and magnetics.
In the deposition process, the film is deposited on the front side of the wafer. However, under certain circumstances, film may be formed on the back side as well as on the edge of the wafer, which poses a problem. That is, the back side and the edge become only partially coated, and these partial coatings tend to peel and flake easily for some types of materials, introducing particulates into the process chamber during deposition and subsequent handling steps. This in turn lowers the quality of the processed wafers, resulting in a lowered yield.
Several approaches have been developed to prevent such partial coating on the back side and the edge, some of which are described in the above-referenced U.S. Pat. No. 5,578,532. According to one such approach, as shown in FIG. 1, a wafer 20 is placed on a chuck 200, and a carrier ring 30 is placed on top of wafer 20, covering the wafer's edge. In this case carrier ring 30 is an exclusion gas ring. The exclusion gas is introduced beneath the wafer and flows around the edge of the wafer 20 and through a gap between the exclusion ring 30 and the wafer 20 to prohibit a film from forming on the edge and the backside of wafer 20. For this process to be successful, the wafer 20 must be centered accurately with respect to both the chuck 200 and the exclusion ring 30.
FIG. 2A is a top view of an illustrative chuck 200, containing integral gas lines and gas grooves to facilitate the introduction of the exclusion gas from within chuck block 202. An annular gas groove 210 is provided within a peripheral region 211 of top surface 15 of chuck block 202. Gas groove 210 intersects ten (10) radial gas lines 212a-212j for distributing gas to the backside of the wafer to be processed. Radial gas lines 212a-212j are shown as dashed lines in FIG. 2A, and selectively in the cross sectional view of FIG. 2B. Gas lines 212a-212j extend sufficiently into chuck block 202 to intersect a respective one of the ten vertical bores 216a-216j, which extend from the bottom surface of chuck block 202 (FIG. 2B). Gas lines 212a-212j are plugged by respective plugs 218a-218j.
A second set of radial gas lines 214a-214c are bored in chuck block 202 for distributing backside gas to holes 107a-107c (FIG. 2C), which accommodate lift pins (not shown) which are used to lift the wafer from the surface 15 of the chuck 200. Radial gas lines 214a-214c are likewise shown as dashed lines in FIG. 2A, and selectively in the cross sectional view of FIG. 2C. Gas lines 214a-214c are plugged by respective plugs 222a-222c.
Holes 205a-205h, radial grooves 206a-206h and 209a-209q, and circular grooves 208a-208c form an interconnected system for providing a vacuum to clamp the wafer to the top surface 15 of chuck 200. Circular grooves 208a-208c and 210 are concentric, and groove 210 for the exclusion gas is located outside of the outermost groove 208c for the vacuum clamping system.
For the exclusion gas to perform satisfactorily, wafer 20 must be accurately centered with respect to the circular exclusion gas groove 210. If the wafer is misaligned and part of it is thereby placed beyond the influence of the deposition control gas from the groove 210, partial film coating on the back side and the edge cannot be prevented.
Moreover, if the wafer is misaligned, a part of the front side may remain uncoated, or the thickness of the coating may be uneven. An important objective in wafer processing is to produce a processed wafer whose front side coating is as uniform as possible. Proper wafer centering before deposition is a key to achieving this objective.
Two basic techniques have been employed for centering wafers on a chuck: active and passive. The active technique typically utilizes a robot arm to place a wafer on a chuck. This is expensive and requires electricity to drive the robot motor. It is also difficult to implement, since slip, inertia and other factors causing inaccurate positioning have to be prevented over an extended period of repetition. Moreover, the circuitry in a robot arm will not withstand the caustic environment in the process chamber.
Passive techniques typically allow a wafer to settle into a centered position by its own weight. One example is illustrated in FIGS. 3A, 3B and 3C. A circular ramp 17 is slanted toward the center of the chuck's top surface 15. FIGS. 3B and 3C show the cross-section of ramp 17. The reference label 3B in FIG. 3A indicates the cross-section shown in FIGS. 3B and 3C. FIG. 3B shows a cross-sectional view of the ramp 17 with an uncentered wafer 20 which straddles a part of ramp 17. Wafer 20 is supposed to slide down ramp 17 to the centered position shown in FIG. 3C.
A problem with this type of arrangement is that friction between the wafer's edge and the ramp may prevent the wafer from sliding. This friction can increase as the surfaces inside a process chamber become coated with the deposition material.
The inside of the process chamber is subjected to a very caustic environment, since a corrosive cleanser, e.g., fluorine, is typically used to eliminate any remnants of deposition material inside the process chamber. Therefore, a wafer centering mechanism should also be anticaustic.
Accordingly, a wafer centering mechanism that is effective, inexpensive to implement and resistant to a caustic environment is desired.